1. Field of the Invention
The present invention relates to subterranean treatment operations, and more particularly, to treatment fluids comprising water and a formate ester, and methods of using these treatment fluids in subterranean formations.
2. Background of the Invention Technology
The production of fluids (e.g., oil and gas) from subterranean formations may often be enhanced by stimulating a region of the formation surrounding a well bore. Where the subterranean formation comprises an acid-soluble component such as calcium carbonate (e.g., limestone), or calcium magnesium carbonate (e.g., dolomite), for example, stimulation is often achieved by contacting the formation with a treatment fluid comprising an acid source, a procedure often referred to as “acidizing” the formation. For example, where formic acid contacts and reacts with a formation comprising calcium carbonate, the calcium carbonate is consumed to produce water and calcium formate, a salt. After the acidization is completed, the water and salts dissolved therein may be recovered by producing them to the surface, e.g., “flowing back” the well, thereby leaving a desirable amount of voids within the formation, which enhances the formation's permeability and increases the rate at which hydrocarbons may subsequently be produced from the formation.
One method of acidizing, known as “fracture acidizing,” comprises injecting such treatment fluid into the formation at a pressure sufficient to create or extend a fracture within the subterranean formation. Once the fracture has been created or extended, the presence of the acid source may “etch” the surface of the formation along the length of the fracture, e.g., the acid source reacts with and consumes a portion of the formation, thereby “etching” channels into the formation. When the fracture closes at the completion of the treatment, a desirable amount of channels remain within the formation, thereby enhancing the permeability of the formation by providing large pathways for hydrocarbons to flow from the formation to the well bore, whereupon the hydrocarbons may be produced to the surface and recovered. Another method of acidizing, known as “matrix acidizing,” comprises injecting the treatment fluid into the formation at a pressure below that which would create or extend a fracture within the subterranean formation. Accordingly, in matrix acidizing, the acid source within the treatment fluid is permitted to contact and react with the matrix of the subterranean formation so as to enhance its permeability, but the formation is not fractured.
Acidizing treatments such as those described above may be problematic, however, because the introduction of an acid to a subterranean formation comprising an acid-soluble component (for example, calcium carbonate) generally results in consumption of the acid near the well bore, rather than deeper penetration into the pores or fractures of the subterranean formation. Where this occurs, it is generally thought to be undesirable in that it may fail to achieve a main goal of acidizing, which is the creation of high conductivity channels extending from the well bore as far into the subterranean formation as needed to achieve the desired degree of stimulation or damage removal. In such circumstances, the operator may often modify the acid source in such a way as to facilitate such contact. For example, an operator may increase the viscosity of the acid before introducing it into the subterranean formation, e.g., by using a treatment fluid comprising a gelled or emulsified form of the acid. The use of a gelled or emulsified acid may be expensive, due to the high cost of viscosifiers which are also resistant to acid exposure. Additionally, the use of a gelled or emulsified acid may be problematic because of the difficulty in removing any residual acid which may undesirably remain in the subterranean formation after application of the gelled or emulsified acid therein. Or, an operator may elect to use a weak acid in conjunction with a conjugate base, wherein the pH of the acid is increased, which may delay the rate at which the acid reacts with the formation until such point as the acid has penetrated further into the formation. This may also be problematic, because such a fluid will have less acid capacity for a given fluid volume.
Another approach has involved the use of a treatment fluid comprising an aqueous solution of an ester of acetic acid (for example, glycerol triacetate) and a lipase-type enzyme, which enzyme may facilitate cleaving the ester and releasing acetic acid so as to acidize the subterranean formation. However, the use of such treatment fluids may be problematic for reasons such as the price of the enzyme, and the cost and difficulty in providing storage for the enzyme (particularly in tropical and desert locations). Further, the use of such treatment fluids may be limited to applications in subterranean formations having a temperature at about or below 212° F. Also, the presence in such treatment fluids of certain commonly-used additives may risk denaturing the enzyme. Additionally, acetic acid is a relatively weak acid, which may adversely affect the efficiency of some applications of treatment fluids comprising a source of acetic acid. For example, an experiment wherein a source of calcium carbonate was contacted with glycerol diacetate showed that after nine days, only about 20% of the theoretically available acetic acid had reacted with the calcium carbonate.